I. General
The present invention is concerned in general with circuit arrangements which alter the dynamic range of audio signals, namely compressors which compress the dynamic range and expanders which expand the dynamic range.
Compressors and expanders are normally used together (a compander system) to effect noise reduction; the signal is compressed before transmission or recording and expanded after reception or playback from the transmission channel. However, compressors may be used alone to reduce the dynamic range, e.g., to suit the capacity of a transmission channel, without subsequent expansion when the compressed signal is adequate for the end purpose. In addition, compressors alone are used in certain products, especially audio products which are intended only to transmit or record compressed broadcast or pre-recorded signals. Expanders alone are used in certain products, especially audio products which are intended only to receive or play back already compressed broadcast or pre-recorded signals. In certain products, a single device is often configured for switchable mode operation as a compressor to record signals and as an expander to play back compressed broadcast or pre-recorded signals.
II. Adaptiveness: the Least Treatment Principle
A long sought after goal in the design of compressors, expanders and companding type noise-reduction systems is a high degree of adaptiveness of the compressor and expander to applied signals. A design philosophy embodying this goal has provided the foundation for a series of successful developments and refinements by Ray M. Dolby beginning in 1965 with A-type noise reduction and culminating recently in the Spectral Recording (SR) process, a highly adaptive signal processing system for high quality professional audio recording.
The underlying design philosophy, which was recently expressed as the "Least Treatment Principle"("The Spectral Recording Process" by Ray Dolby, J. Audio Eng. Soc., 35, No. 3, 1987 March, pp. 99-118 at page 100) has its roots in the concept of "conformal equalization" mentioned in the U.K. Provisional Specification No. 43136 filed Oct. 11, 1965 of Ray M. Dolby. According to the Least Treatment Principle, the best treatment of the signal is the least treatment. The operating goal of the encoder is to provide fixed, predetermined gains for all frequency components of the signal, with corresponding attenuations in the decoder. If a large signal component appears at a particular frequency or frequencies, then the gains should be reduced at those frequencies only, in accordance with predetermined compression laws for restoration of the signal during decoding. In other words, the compressor should try to keep all signal components fully boosted at all times. When the boosting must be cut back at a particular frequency, the effect should not be extended to low-level signal components at other frequencies.